Search for new phenomena in events with same-charge leptons and $b$-jets in $pp$ collisions at $\sqrt{s}= 13$ TeV with the ATLAS detector

TL;DR

This paper reports a search for new physics phenomena involving same-charge leptons and $b$-jets in proton-proton collisions at 13 TeV, setting limits on various exotic particles and processes with no significant excess observed.

Contribution

It provides the first limits on vector-like quarks, four-top-quark production, and same-sign top-quark pairs in events with same-charge leptons and $b$-jets at 13 TeV.

Findings

No significant excess observed in the data.

Set 95% CL mass limits for vector-like quarks around 1 TeV.

Established upper limits on four-top-quark and same-sign top-quark production cross-sections.

Abstract

A search for new phenomena in events with two same-charge leptons or three leptons and jets identified as originating from $b$-quarks in a data sample of 36.1 fb$^{-1}$ of $pp$ collisions at $\sqrt{s}= 13$ TeV recorded by the ATLAS detector at the Large Hadron Collider is reported. No significant excess is found and limits are set on vector-like quark, four-top-quark, and same-sign top-quark pair production. The observed (expected) 95% CL mass limits for a vector-like $T$- and $B$-quark singlet are $m_T > 0.98$ $(0.99)$ TeV and $m_B > 1.00$ $(1.01)$ TeV respectively. Limits on the production of the vector-like $T_{5/3}$-quark are also derived considering both pair and single production; in the former case the lower limit on the mass of the $T_{5/3}$-quark is (expected to be) 1.19 (1.21) TeV. The Standard Model four-top-quark production cross-section upper limit is (expected to be) 69 (29) fb. Constraints are also set on exotic four-top-quark production models. Finally, limits are set on same-sign top-quark pair production. The upper limit on $uu \to tt$ production is (expected to be) 89 (59) fb for a mediator mass of 1 TeV, and a dark-matter interpretation is also derived, excluding a mediator of 3 TeV with a dark-sector coupling of 1.0 and a coupling to ordinary matter above 0.31.